The present invention relates to apparatus for electrically testing insulation coatings on wire to detect undesired faults or discontinuities. More particularly, it relates to apparatus for detecting faults in enamel insulation coatings on relatively small magnet wire while the wire is moving at relatively high speeds. Such fault tests are advantageously made immediately after the wire has passed through the enamel curing oven immediately prior to being wound on spools.
Manufacturing standards have been established setting a maximum fault-rate limit of 3, 5 and 10 faults per one hundred feet for triple, heavy and single film insulation coatings, respectively. To have assurance that these industry standards are complied with, it is essential that all manufacturing production of such coated wire be continuously tested. It is desirable of course to make the continuity tests as rapidly as possible; on the other hand, as a practical matter it is difficult to sense and record extremely short duration fault currents generated by small pinhole defects traveling at high speeds past the test instrument. Reliable testing is further complicated by the fact that measuring sensitivity must be increased as a function of insulation thickness and, at the same time, current flow through the insulation voids must be limited to prevent insulation destruction throughout the range of D-C high voltages that are utilized.
One of the time consuming problems encountered with the use of prior art insulation testing devices is that of counting the detected faults (normally recorded on a chart recorder) in order to evaluate production quality based on fault-rate industry standards.
It is a principal object of the present invention to provide a reliable insulation continuity tester which can be quickly adjusted to measure faults in a wide range of coating thickness, and at the same time automatically actuate an alarm indicator when the fault rate exceeds a preselected limit.